This invention relates to silicon solar cells, and more particularly monolithic silicon multijunction solar cells with III-V or II-VI tunnel junctions to connect solar cells of a stack.
Silicon is the least expensive semiconductor material available for solar cells. Silicon solar cell conversion efficiency has been continuously improved from about 5% in 1956 to more than 16% at the present time. Improvements both in materials technology and in cell processing has been largely responsible for this. However, a compromise between shallower junctions and lower series resistance of the front diffused region soon reached its limits. (Losses in the diffused region as well as the metal grid are proportional to the square of the current). Similarly, a limit has been reached in a compromise between more absorption with thicker cells and more collection at the front junction. Such a compromise needs longer minority carrier diffusion lengths.
The quest for higher efficiency has led to stacking solar cells. Because of improvements in techniques of epitaxial silicon growth, with finer control of doping possible, a multilayered silicon structure of two or more p/n junctions can be produced with heavily doped n.sup.+ /p.sup.+ tunnel junction between them acting as low resistance connections. The stack is characterized by high open circuit voltage, which is the sum of the open circuit voltages of the individual cells thus stacked.
One would think that heavily doped silicon layers could be used as tunneling junctions in multijunction silicon solar cells. However, analysis has shown that the interface recombination velocity at the silicon tunneling junction is greater than 10.sup.4 cm/sec, which degrades performance below that of a conventional monojunction silicon cell. To minimize this recombination loss, the present invention combines silicon (group IV semiconductor) with GaP (III-V) or ZnS (II-VI) compounds which are uniquely suited for the tunneling junction in the multijunction silicon solar cells.
Because of the technological advancements of silicon compared with III-V semiconductor compounds, a number of opto-electronic devices are made from silicon wafers. The present invention provides a viable high voltage source which has excellent compatibility with other devices fabricated on silicon chips. This compatibility may not be achievable with solar cells based on III-V compounds.